The present invention relates to composite membranes based on porous semipermeable membrane substrates, and to processes using such composite membranes.
Composite membranes, in which a thin crosslinked hydrophilic film is chemically bound to a thicker more porous membrane, are known, see e.g. EP 25973 and 26399. These membranes require reactive groups in the substrate to chemically bind the thin film thereto, and crosslinking e.g. with reactive dye. Without this chemical binding, the coated polymer crosslinked with reactive dye had a relatively short life, especially at extremes of pH and temperature, and also suffered from the disadvantage that it was soluble in various organic solvents such as N-methylpyrrolidone (NMP), dimethylformamide (DMF) or dimethylsuloxide (DMSO). One approach to coating an ultrafiltration membrane with hydrophilic polymers, for improving rejection to undesired solutes, is described in U.S. Pat. No. 4,125,462. The coating polymers are water-soluble amine group containing polymers which are adsorbed physically, without any attempt at crosslinking or binding the coated layers.
An object of the present invention is to provide composite membranes of superior flux and separation capacity. Other objects of the invention are processes utilizing such membranes for concentrating or purifying liquids. Further objects of the invention will appear from the description which follows.